source: draft-ietf-httpbis/latest/p6-cache.xml @ 834

Last change on this file since 834 was 834, checked in by mnot@…, 9 years ago

Clarify caching of authenticated responses by shared caches (see #174)

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "July">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc rfcedstyle="yes"?>
46<?rfc-ext allow-markup-in-artwork="yes" ?>
47<?rfc-ext include-references-in-index="yes" ?>
48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
50<front>
51
52  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
53
54  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
55    <organization abbrev="Day Software">Day Software</organization>
56    <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69  </author>
70
71  <author fullname="Jim Gettys" initials="J." surname="Gettys">
72    <organization>One Laptop per Child</organization>
73    <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@laptop.org</email>
82      <uri>http://www.laptop.org/</uri>
83    </address>
84  </author>
85
86  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
87    <organization abbrev="HP">Hewlett-Packard Company</organization>
88    <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99  </author>
100
101  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
102    <organization abbrev="Microsoft">Microsoft Corporation</organization>
103    <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113  </author>
114
115  <author fullname="Larry Masinter" initials="L." surname="Masinter">
116    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
117    <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128  </author>
129
130  <author fullname="Paul J. Leach" initials="P." surname="Leach">
131    <organization abbrev="Microsoft">Microsoft Corporation</organization>
132    <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141  </author>
142
143  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
144    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145    <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158  </author>
159
160  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
161    <organization abbrev="W3C">World Wide Web Consortium</organization>
162    <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174  </author>
175
176  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
177    <address>
178      <email>mnot@mnot.net</email>
179      <uri>http://www.mnot.net/</uri>
180    </address>
181  </author>
182
183  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
184    <organization abbrev="greenbytes">greenbytes GmbH</organization>
185    <address>
186      <postal>
187        <street>Hafenweg 16</street>
188        <city>Muenster</city><region>NW</region><code>48155</code>
189        <country>Germany</country>
190      </postal>
191      <phone>+49 251 2807760</phone>
192      <facsimile>+49 251 2807761</facsimile>
193      <email>julian.reschke@greenbytes.de</email>
194      <uri>http://greenbytes.de/tech/webdav/</uri>
195    </address>
196  </author>
197
198  <date month="&ID-MONTH;" year="&ID-YEAR;" />
199  <workgroup>HTTPbis Working Group</workgroup>
200
201<abstract>
202<t>
203  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
204  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
205  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
206  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
207  fields that control cache behavior or indicate cacheable response messages.
208</t>
209</abstract>
210
211<note title="Editorial Note (To be removed by RFC Editor)">
212<t>
213  Discussion of this draft should take place on the HTTPBIS working group mailing list
214  (ietf-http-wg@w3.org). The current issues list is at <eref
215  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
216  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
217</t>
218<t>
219  The changes in this draft are summarized in <xref target="changes.since.09"/>.
220</t>
221</note>
222
223  </front>
224  <middle>
225
226<section anchor="caching" title="Introduction">
227<t>
228  HTTP is typically used for distributed information systems, where performance can be
229  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
230  caching and reusing response messages.
231</t>
232
233<section anchor="intro.purpose" title="Purpose">
234<iref item="cache" />
235<t>
236  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
237  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
238  in order to reduce the response time and network bandwidth consumption on future,
239  equivalent requests. Any client or server may include a cache, though a cache cannot be
240  used by a server that is acting as a tunnel.
241</t>
242<t>
243  Caching would be useless if it did not significantly improve performance. The goal of
244  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
245  some cases, a stored response can be reused without the need for a network request,
246  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
247  (see <xref target="expiration.model" />). Even when a new request is required, it is often
248  possible to reuse all or parts of the payload of a prior response to satisfy the request,
249  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
250  purpose (see <xref target="validation.model" />).
251</t>
252</section>
253
254<section anchor="intro.terminology" title="Terminology">
255<t>
256  This specification uses a number of terms to refer to the roles played by participants
257  in, and objects of, HTTP caching.
258</t>
259<t>
260  <iref item="cacheable" />
261  <x:dfn>cacheable</x:dfn>
262  <list>
263    <t>A response is cacheable if a cache is allowed to store a copy of the response message
264      for use in answering subsequent requests. Even when a response is cacheable, there may
265      be additional constraints on whether a cache can use the cached copy to satisfy a
266      particular request.</t>
267  </list>
268</t>
269<t>
270  <iref item="explicit expiration time" />
271  <x:dfn>explicit expiration time</x:dfn>
272  <list>
273    <t>The time at which the origin server intends that an entity should no longer be
274      returned by a cache without further validation.</t>
275  </list>
276</t>
277<t>
278  <iref item="heuristic expiration time" />
279  <x:dfn>heuristic expiration time</x:dfn>
280  <list>
281    <t>An expiration time assigned by a cache when no explicit expiration time is
282    available.</t>
283  </list>
284</t>
285<t>
286  <iref item="age" />
287  <x:dfn>age</x:dfn>
288  <list>
289    <t>The age of a response is the time since it was sent by, or successfully validated
290      with, the origin server.</t>
291  </list>
292</t>
293<t>
294  <iref item="first-hand" />
295  <x:dfn>first-hand</x:dfn>
296  <list>
297    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
298    0.</t>
299  </list>
300</t>
301<t>
302  <iref item="freshness lifetime" />
303  <x:dfn>freshness lifetime</x:dfn>
304  <list>
305    <t>The length of time between the generation of a response and its expiration time. </t>
306  </list>
307</t>
308<t>
309  <iref item="fresh" />
310  <x:dfn>fresh</x:dfn>
311  <list>
312    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
313  </list>
314</t>
315<t>
316  <iref item="stale" />
317  <x:dfn>stale</x:dfn>
318  <list>
319    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
320  </list>
321</t>
322<t>
323  <iref item="validator" />
324  <x:dfn>validator</x:dfn>
325  <list>
326    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
327      out whether a stored response is an equivalent copy of an entity.</t>
328  </list>
329</t>
330<t anchor="shared.and.non-shared.caches">
331  <iref item="validator" />
332  <x:dfn>shared cache</x:dfn>
333  <list>
334    <t>A cache that is accessible to more than one user. A non-shared cache is
335      dedicated to a single user.</t>
336  </list>
337</t>
338</section>
339
340<section anchor="intro.requirements" title="Requirements">
341<t>
342  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
343  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
344  described in <xref target="RFC2119" />.
345</t>
346<t>
347  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
348  or &REQUIRED; level requirements for the protocols it implements. An implementation
349  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
350  requirements for its protocols is said to be "unconditionally compliant"; one that
351  satisfies all the &MUST; level requirements but not all the &SHOULD; level
352  requirements for its protocols is said to be "conditionally compliant."
353</t>
354</section>
355
356<section title="Syntax Notation" anchor="notation">
357  <x:anchor-alias value="ALPHA"/>
358  <x:anchor-alias value="CR"/>
359  <x:anchor-alias value="DIGIT"/>
360  <x:anchor-alias value="DQUOTE"/>
361  <x:anchor-alias value="LF"/>
362  <x:anchor-alias value="OCTET"/>
363  <x:anchor-alias value="SP"/>
364  <x:anchor-alias value="VCHAR"/>
365  <x:anchor-alias value="WSP"/>
366<t>
367  This specification uses the ABNF syntax defined in &notation; (which
368  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
369  <xref target="collected.abnf"/> shows the collected ABNF, with the list
370  rule expanded.
371</t>
372<t>
373  The following core rules are included by
374  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
375  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
376  DIGIT (decimal 0-9), DQUOTE (double quote),
377  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
378  OCTET (any 8-bit sequence of data), SP (space),
379  VCHAR (any visible USASCII character),
380  and WSP (whitespace).
381</t>
382
383<section title="Core Rules" anchor="core.rules">
384  <x:anchor-alias value="quoted-string"/>
385  <x:anchor-alias value="token"/>
386  <x:anchor-alias value="OWS"/>
387<t>
388  The core rules below are defined in &basic-rules;:
389</t>
390<figure><artwork type="abnf2616">
391  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
392  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
393  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
394</artwork></figure>
395</section>
396
397<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
398  <x:anchor-alias value="field-name"/>
399  <x:anchor-alias value="HTTP-date"/>
400  <x:anchor-alias value="port"/>
401  <x:anchor-alias value="pseudonym"/>
402  <x:anchor-alias value="uri-host"/>
403<t>
404  The ABNF rules below are defined in other parts:
405</t>
406<figure><!--Part1--><artwork type="abnf2616">
407  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
408  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
409  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
410  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
411  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
412</artwork></figure>
413</section>
414
415</section>
416</section>
417
418<section anchor="caching.overview" title="Cache Operation">
419
420<section anchor="response.cacheability" title="Response Cacheability">
421<t>
422  A cache &MUST-NOT; store a response to any request, unless:
423  <list style="symbols">
424    <t>The request method is understood by the cache and defined as being cacheable, and</t>
425    <t>the response status code is understood by the cache, and</t>
426    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
427       appear in request or response headers, and</t>
428    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
429       does not appear in the response, if the cache is shared, and</t>
430    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
431       the cache is shared, unless the response explicitly allows it (see <xref
432       target="caching.authenticated.responses" />), and</t>
433    <t>the response either:
434      <list style="symbols">
435        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
436        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
437        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
438        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
439        <t>has a status code that can be served with heuristic freshness (see <xref
440           target="heuristic.freshness" />).</t>
441      </list>
442    </t>
443  </list>
444</t>
445<t>
446  In this context, a cache has "understood" a request method or a response status
447  code if it recognises it and implements any cache-specific behaviour. In
448  particular, 206 Partial Content responses cannot be cached by an
449  implementation that does not handle partial content
450  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
451</t>
452<t>
453  Note that in normal operation, most caches will not store a response that has neither a
454  cache validator nor an explicit expiration time, as such responses are not usually
455  useful to store. However, caches are not prohibited from storing such responses.
456</t>
457
458<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
459<t>
460  A cache that receives an incomplete response (for example, with fewer bytes of data
461  than specified in a Content-Length header) can store the response, but &MUST; 
462  treat it as a partial response &partial;. Partial responses
463  can be combined as described in &combining-byte-ranges;; the result might be a
464  full response or might still be partial. A cache &MUST-NOT; return a partial
465  response to a client without explicitly marking it as such using the 206 (Partial
466  Content) status code.
467</t>
468<t>
469  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
470  incomplete or partial responses.
471</t>
472</section>
473
474</section>
475
476
477<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
478<t>
479  For a presented request, a cache &MUST-NOT; return a stored response, unless:
480  <list style="symbols">
481    <t>The presented Effective Request URI (&effective-request-uri;) and that of the stored response match, and</t>
482    <t>the request method associated with the stored response allows it to be
483      used for the presented request, and</t>
484    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
485      target="caching.negotiated.responses" />), and</t>
486    <t>the presented request and stored response are free from directives that would prevent
487      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
488      and</t>
489    <t>the stored response is either:
490      <list style="symbols">
491        <t>fresh (see <xref target="expiration.model" />), or</t>
492        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
493        <t>successfully validated (see <xref target="validation.model" />).</t>
494      </list>
495    </t>
496  </list>
497</t>
498<t>
499  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
500</t>
501<t>
502  When a stored response is used to satisfy a request, caches &MUST; include a
503  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
504  current_age; see <xref target="age.calculations" />.
505  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
506</t>
507<t>
508  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
509  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
510  corresponding response.
511</t>
512<t>
513  Also, note that unsafe requests might invalidate already stored responses; see
514  <xref target="invalidation.after.updates.or.deletions" />.
515</t>
516<t>
517  Caches &MUST; use the most recent response (as determined by the Date header) when
518  more than one suitable response is stored. They can also forward a request with
519  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
520  use.
521</t>
522</section>
523
524<section anchor="expiration.model" title="Freshness Model">
525<t>
526  When a response is "fresh" in the cache, it can be used to satisfy subsequent
527  requests without contacting the origin server, thereby improving efficiency.
528</t>
529<t>
530  The primary mechanism for determining freshness is for an origin server to provide an
531  explicit expiration time in the future, using either the Expires header (<xref
532  target="header.expires" />) or the max-age response cache directive (<xref
533  target="cache-response-directive" />). Generally, origin servers will assign future
534  explicit expiration times to responses in the belief that the entity is not likely to
535  change in a semantically significant way before the expiration time is reached.
536</t>
537<t>
538  If an origin server wishes to force a cache to validate every request, it can
539  assign an explicit expiration time in the past. This means that the response is always
540  stale, so that caches should validate it before using it for subsequent requests.
541  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
542</t>
543<t>
544  Since origin servers do not always provide explicit expiration times, HTTP caches may
545  also assign heuristic expiration times when they are not specified, employing algorithms that
546  use other header values (such as the Last-Modified time) to estimate a plausible
547  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
548  impose worst-case constraints on their results.
549</t>
550<figure>
551<preamble>
552  The calculation to determine if a response is fresh is:
553</preamble>
554<artwork type="code">
555   response_is_fresh = (freshness_lifetime &gt; current_age)
556</artwork>
557</figure>
558<t>
559  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
560  the current_age is defined in <xref target="age.calculations" />.
561</t>
562<t>
563  Additionally, clients may need to influence freshness calculation. They can do this using
564  several request cache directives, with the effect of either increasing or loosening
565  constraints on freshness. See <xref target="cache-request-directive" />.
566</t>
567<t>
568  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
569  freshness.</cref>
570</t>
571<t>
572  Note that freshness applies only to cache operation; it cannot be used to force a user agent
573  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
574  the difference between caches and history mechanisms.
575</t>
576
577<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
578<t>
579  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
580  response by using the first match of:
581  <list style="symbols">
582    <t>If the cache is shared and the s-maxage response cache directive (<xref
583      target="cache-response-directive" />) is present, use its value, or</t>
584    <t>If the max-age response cache directive (<xref target="cache-response-directive"
585      />) is present, use its value, or</t>
586    <t>If the Expires response header (<xref target="header.expires" />) is present, use
587      its value minus the value of the Date response header, or</t>
588    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
589      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
590  </list>
591</t>
592<t>
593  Note that this calculation is not vulnerable to clock skew, since all of the
594  information comes from the origin server.
595</t>
596
597<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
598<t>
599  If no explicit expiration time is present in a stored response that has a status code
600  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
601  calculated. Heuristics &MUST-NOT; be used for other response status codes.
602</t>
603<t>
604  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
605  attach a Warning header with a 113 warn-code to the response if its current_age is
606  more than 24 hours and such a warning is not already present.
607</t>
608<t>
609  Also, if the response has a Last-Modified header (&header-last-modified;), the
610  heuristic expiration value &SHOULD; be no more than some fraction of the interval
611  since that time. A typical setting of this fraction might be 10%.
612</t>
613<x:note>
614  <t>
615    <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
616    required that caches do not calculate heuristic freshness for URLs with
617    query components (i.e., those containing '?'). In practice, this has not
618    been widely implemented. Therefore, servers are encouraged to send explicit
619    directives (e.g., Cache-Control: no-cache) if they wish to preclude
620    caching.
621  </t>
622</x:note>
623</section>
624</section>
625
626<section anchor="age.calculations" title="Calculating Age">
627<t>
628  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
629  message when obtained from a cache. The Age field value is the cache's estimate of the
630  amount of time since the response was generated or validated by the origin server. In
631  essence, the Age value is the sum of the time that the response has been resident in
632  each of the caches along the path from the origin server, plus the amount of time it has
633  been in transit along network paths.
634</t>
635<t>
636  The following data is used for the age calculation:
637</t>
638<t>
639  <x:dfn>age_value</x:dfn>
640  <list>
641    <t>
642      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
643      in a form appropriate for arithmetic operation; or 0, if not available.
644    </t>
645  </list>
646</t>
647<t>
648  <x:dfn>date_value</x:dfn>
649  <list>
650    <t>
651      HTTP/1.1 requires origin servers to send a Date header, if possible,
652      with every response, giving the time at which the response was generated.
653      The term "date_value" denotes the value of the Date header, in a form
654      appropriate for arithmetic operations. See &header-date; for the definition
655      of the Date header, and for requirements regarding responses without a
656      Date response header.
657    </t>
658  </list>
659</t>
660<t>
661  <x:dfn>now</x:dfn>
662  <list>
663    <t>
664      The term "now" means "the current value of the clock at the host
665      performing the calculation". Hosts that use HTTP, but especially hosts
666      running origin servers and caches, &SHOULD; use NTP
667      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
668      clocks to a globally accurate time standard.
669    </t>
670  </list>
671</t>
672<t>
673  <x:dfn>request_time</x:dfn>
674  <list>
675    <t>
676      The current value of the clock at the host at the time the request
677      resulting in the stored response was made.
678    </t>
679  </list>
680</t>
681<t>
682  <x:dfn>response_time</x:dfn>
683  <list>
684    <t>
685      The current value of the clock at the host at the time the response was
686      received.
687    </t>
688  </list>
689</t>
690<t>
691  A response's age can be calculated in two entirely independent ways:
692  <list style="numbers">
693    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
694      origin server's clock. If the result is negative, the result is replaced by zero.</t>
695    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
696      note this value &MUST; be interpreted relative to the time the
697      request was initiated, not the time that the response was received.</t>
698  </list>
699</t>
700<figure>
701<artwork type="code">
702  apparent_age = max(0, response_time - date_value);
703
704  response_delay = response_time - request_time;
705  corrected_age_value = age_value + response_delay; 
706</artwork>
707</figure>
708<figure>
709<preamble>These are combined as</preamble>
710<artwork type="code">
711  corrected_initial_age = max(apparent_age, corrected_age_value);
712</artwork></figure>
713<t>
714  The current_age of a stored response can then be calculated by adding the amount of
715  time (in seconds) since the stored response was last validated by the origin server to
716  the corrected_initial_age.
717</t>
718<figure><artwork type="code">
719  resident_time = now - response_time;
720  current_age = corrected_initial_age + resident_time;
721</artwork></figure>
722</section>
723
724<section anchor="serving.stale.responses" title="Serving Stale Responses">
725<t>
726  A "stale" response is one that either has explicit expiry information or is allowed to
727  have heuristic expiry calculated, but is not fresh according to the calculations in
728  <xref target="expiration.model" />.
729</t>
730<t>
731  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
732  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
733  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
734  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
735</t>
736<t>
737  Caches &SHOULD-NOT; return stale responses unless they are
738  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
739  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
740</t>
741<t>
742  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
743  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
744  the cache is disconnected.
745</t>
746<t>
747  If a cache receives a first-hand response (either an entire response, or a 304 (Not
748  Modified) response) that it would normally forward to the requesting client, and the
749  received response is no longer fresh, the cache &SHOULD; forward it to the
750  requesting client without adding a new Warning (but without removing any existing
751  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
752  that response became stale in transit.
753</t>
754</section>
755</section>
756
757<section anchor="validation.model" title="Validation Model">
758<t>
759  When a cache has one or more stored responses for a requested URI, but cannot 
760  serve any of them (e.g., because they are not fresh, or one cannot be selected;
761  see <xref target="caching.negotiated.responses"/>),
762  it can use the conditional request mechanism &conditional; in the forwarded
763  request to give the origin server an opportunity to both select a valid stored
764  response to be used, and to update it. This process is known as "validating"
765  or "revalidating" the stored response.
766</t>
767<t>
768  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
769  header whose value is that of the Last-Modified header from the selected
770  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
771</t>
772<t>
773  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
774  is that of the ETag header(s) from all responses stored for the requested URI,
775  if present. However, if any of the stored responses contains only partial
776  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
777  field unless the request is for a range that would be fully satisfied by
778  that stored response.
779</t>
780<t>
781  A 304 (Not Modified) response status code indicates that the stored
782  response can be updated and reused; see <xref target="combining.headers"/>.
783</t>
784<t>
785  A full response (i.e., one with a response body) indicates that none 
786  of the stored responses nominated in the conditional request is
787  suitable. Instead, the full response is used both to satisfy the
788  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
789</t>
790<t>
791  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
792  either forward this response to the requesting client, or act as if the server failed to
793  respond. In the latter case, it &MAY; return a previously stored response (see <xref
794  target="serving.stale.responses" />).
795</t>
796</section>
797
798<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
799<t>
800  Because unsafe methods (&safe-methods;) have the potential for changing state on the
801  origin server, intervening caches can use them to keep their contents
802  up-to-date.
803</t>
804<t>
805  The following HTTP methods &MUST; cause a cache to invalidate the Effective Request URI (&effective-request-uri;) as well
806  as the URI(s) in the Location and Content-Location headers (if present):
807  <list style="symbols">
808    <t>PUT</t>
809    <t>DELETE</t>
810    <t>POST</t>
811  </list>
812</t>
813<t>
814  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
815  be performed if the host part of that URI differs from the host part in the Effective Request URI (&effective-request-uri;).
816  This helps prevent denial of service attacks.
817</t>
818<t>
819  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
820</t>
821<t>
822  A cache that passes through requests for methods it does not understand &SHOULD;
823  invalidate the Effective Request URI (&effective-request-uri;).
824</t>
825<t>
826  Here, "invalidate" means that the cache will either remove all stored responses related
827  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
828  before they can be returned in response to a subsequent request.
829</t>
830<t>
831  Note that this does not guarantee that all appropriate responses are invalidated. For
832  example, the request that caused the change at the origin server might not have gone
833  through the cache where a response is stored.
834</t>
835<t>
836  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
837</t>
838</section>
839
840<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
841
842<t>Shared caches &MUST-NOT; use a cached response to a request with an Authorization header (&header-authorization;) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.</t>
843
844<t>In this specification, the following Cache-Control response directives (<xref target="cache-response-directive"/>) have such an effect: must-revalidate, public, s-maxage.</t>
845
846<t>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.</t>
847</section>
848
849<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
850<t>
851  When a cache receives a request that can be satisfied by a stored response
852  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
853  response unless all of the selecting request-headers nominated by the Vary header match
854  in both the original request (i.e., that associated with the stored response),
855  and the presented request.
856</t>
857<t>
858  The selecting request-headers from two requests are defined to match
859  if and only if those in the first request can be transformed to those in the
860  second request by applying any of the following:
861  <list style="symbols">
862    <t>
863      adding or removing whitespace, where allowed in the header's syntax
864    </t>
865    <t>
866      combining multiple message-header fields with the same field name (see
867      &header-fields;)
868    </t>
869    <t>
870      normalizing both header values in a way that is known to have identical
871      semantics, according to the header's specification (e.g., re-ordering field values
872      when order is not significant; case-normalization, where values are defined to be
873      case-insensitive)   
874    </t>
875  </list>
876</t>
877<t>
878  If (after any normalisation that may take place) a header field is absent
879  from a request, it can only match another request if it is also absent there.
880</t>
881<t>
882  A Vary header field-value of "*" always fails to match, and subsequent requests to that
883  resource can only be properly interpreted by the origin server.
884</t>
885<t>
886  The stored response with matching selecting request-headers is known as the
887  selected response.
888</t>
889<t>
890  If no selected response is available, the cache &MAY; forward the presented
891  request to the origin server in a conditional request; see <xref target="validation.model"/>.
892</t>
893</section>
894
895<section anchor="combining.headers" title="Combining Responses">
896<t>
897  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
898  (in this section, the "new" response"), it needs to created an updated response by combining
899  the stored response with the new one, so that the updated response can be used to satisfy the request.
900</t>
901<t>
902  If the new response contains an ETag, it identifies the stored 
903  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
904  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
905</t>
906<t>
907  If the status code is 206 (partial content), both the stored and new 
908  responses &MUST; have validators, and those validators &MUST; match using the strong 
909  comparison function (see &weak-and-strong;). Otherwise, the 
910  responses &MUST-NOT; be combined.
911</t>
912<t>
913  The stored response headers are used as those of the updated response, except that
914  <list style="symbols">
915    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
916      &MUST; be deleted from the stored response and the updated response.</t>
917    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
918      response and the updated response.</t>
919    <t>any headers provided in the new response &MUST; replace the corresponding
920      headers from the stored response.</t>
921  </list>
922</t>
923<t>
924  If a header field-name in the new response matches more than one 
925  header in the stored response, all such stored headers &MUST; be replaced.
926</t>
927<t>
928  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
929  stored response in cache. In the case of a 206 response, the combined 
930  entity-body &MAY; be stored.
931</t>
932<t>
933  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
934</t>
935</section>
936
937</section>
938
939<section anchor="header.fields" title="Header Field Definitions">
940<t>
941  This section defines the syntax and semantics of HTTP/1.1 header fields
942  related to caching.
943</t>
944<t>
945  For entity-header fields, both sender and recipient refer to either the client or the
946  server, depending on who sends and who receives the entity.
947</t>
948
949<section anchor="header.age" title="Age">
950  <iref item="Age header" primary="true" x:for-anchor="" />
951  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
952  <x:anchor-alias value="Age"/>
953  <x:anchor-alias value="Age-v"/>
954  <x:anchor-alias value="age-value"/>
955<t>
956  The "Age" response-header field conveys the sender's estimate of the amount
957  of time since the response was generated or successfully validated at the
958  origin server. Age values are calculated as specified in
959  <xref target="age.calculations" />.
960</t>
961<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
962  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
963  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
964</artwork></figure>
965<t anchor="rule.delta-seconds">
966  <x:anchor-alias value="delta-seconds" />
967  Age field-values are non-negative integers, representing time in seconds.
968</t>
969<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
970  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
971</artwork></figure>
972<t>
973  If a cache receives a value larger than the largest positive integer it can represent, or
974  if any of its age calculations overflows, it &MUST; transmit an Age header with a
975  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
976  of at least 31 bits of range.
977</t>
978<t>
979  The presence of an Age header field in a response implies that a response is not
980  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
981  Age header field.
982</t>
983</section>
984
985<section anchor="header.cache-control" title="Cache-Control">
986  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
987  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
988  <x:anchor-alias value="Cache-Control"/>
989  <x:anchor-alias value="Cache-Control-v"/>
990  <x:anchor-alias value="cache-directive"/>
991  <x:anchor-alias value="cache-extension"/>
992  <x:anchor-alias value="cache-request-directive"/>
993  <x:anchor-alias value="cache-response-directive"/>
994<t>
995  The "Cache-Control" general-header field is used to specify directives for
996  caches along the request/response chain. Such cache directives are
997  unidirectional in that the presence of a directive in a request does not
998  imply that the same directive is to be given in the response.
999</t>
1000<t>
1001  HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
1002  defined in this section. See <xref target="cache.control.extensions"/> for
1003  information about how Cache-Control directives defined elsewhere are handled.
1004</t>
1005<x:note>
1006  <t>
1007    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1008    might only implement Pragma: no-cache (see <xref target="header.pragma" />).
1009  </t>
1010</x:note>
1011<t>
1012  Cache directives &MUST; be passed through by a proxy or gateway application,
1013  regardless of their significance to that application, since the directives might be
1014  applicable to all recipients along the request/response chain. It is not possible to
1015  target a directive to a specific cache.
1016</t>
1017<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1018  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1019  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1020
1021  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1022     / <x:ref>cache-response-directive</x:ref>
1023
1024  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1025</artwork></figure>
1026
1027<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1028  <x:anchor-alias value="cache-request-directive" />
1029
1030<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1031  <x:ref>cache-request-directive</x:ref> =
1032       "no-cache"
1033     / "no-store"
1034     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1035     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1036     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1037     / "no-transform"
1038     / "only-if-cached"
1039     / <x:ref>cache-extension</x:ref>
1040</artwork></figure>
1041
1042<t>
1043  <x:dfn>no-cache</x:dfn>
1044  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1045  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1046  <list>
1047    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1048      used to satisfy the request without successful validation on the origin server.</t>
1049  </list>
1050</t>
1051<t>
1052  <x:dfn>no-store</x:dfn>
1053  <iref item="Cache Directives" primary="true" subitem="no-store" />
1054  <iref item="no-store" primary="true" subitem="Cache Directive" />
1055  <list>
1056    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1057      of either this request or any response to it. This directive applies to both
1058      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1059      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1060      and &MUST; make a best-effort attempt to remove the information from volatile
1061      storage as promptly as possible after forwarding it.</t>
1062    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1063      particular, malicious or compromised caches might not recognize or obey this
1064      directive, and communications networks may be vulnerable to eavesdropping.</t>
1065  </list>
1066</t>
1067<t>
1068  <x:dfn>max-age</x:dfn>
1069  <iref item="Cache Directives" primary="true" subitem="max-age" />
1070  <iref item="max-age" primary="true" subitem="Cache Directive" />
1071  <list>
1072    <t>The max-age request directive indicates that the client is willing to accept a
1073      response whose age is no greater than the specified time in seconds. Unless
1074      the max-stale request directive is also present, the client is not willing to accept a stale
1075      response.</t>
1076  </list>
1077</t>
1078<t>
1079  <x:dfn>max-stale</x:dfn>
1080  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1081  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1082  <list>
1083    <t>The max-stale request directive indicates that the client is willing to accept a
1084      response that has exceeded its expiration time. If max-stale is assigned a value,
1085      then the client is willing to accept a response that has exceeded its expiration
1086      time by no more than the specified number of seconds. If no value is assigned to
1087      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
1088  </list>
1089</t>
1090<t>
1091  <x:dfn>min-fresh</x:dfn>
1092  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1093  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1094  <list>
1095    <t>The min-fresh request directive indicates that the client is willing to accept a
1096      response whose freshness lifetime is no less than its current age plus the specified
1097      time in seconds. That is, the client wants a response that will still be fresh for
1098      at least the specified number of seconds.</t>
1099  </list>
1100</t>
1101<t>
1102  <x:dfn>no-transform</x:dfn>
1103  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1104  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1105  <list>
1106    <t>The no-transform request directive indicates that an intermediate cache or proxy
1107      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1108      headers, nor the request entity-body.</t>
1109  </list>
1110</t>
1111<t>
1112  <x:dfn>only-if-cached</x:dfn>
1113  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1114  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1115  <list>
1116    <t>The only-if-cached request directive indicates that the client only wishes to
1117      return a stored response. If it receives this directive, a cache &SHOULD; either
1118      respond using a stored response that is consistent with the other constraints of the
1119      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1120      being operated as a unified system with good internal connectivity, such a request
1121      &MAY; be forwarded within that group of caches.</t>
1122  </list>
1123</t>
1124</section>
1125
1126<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1127  <x:anchor-alias value="cache-response-directive" />
1128
1129<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1130  <x:ref>cache-response-directive</x:ref> =
1131       "public"
1132     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1133     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1134     / "no-store"
1135     / "no-transform"
1136     / "must-revalidate"
1137     / "proxy-revalidate"
1138     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1139     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1140     / <x:ref>cache-extension</x:ref>
1141</artwork></figure>
1142
1143<t>
1144  <x:dfn>public</x:dfn>
1145  <iref item="Cache Directives" primary="true" subitem="public" />
1146  <iref item="public" primary="true" subitem="Cache Directive" />
1147  <list>
1148    <t>The public response directive indicates that the response &MAY; be cached, even
1149      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1150      (See also Authorization, &header-authorization;, for additional details.) </t>
1151  </list>
1152</t>
1153<t>
1154  <x:dfn>private</x:dfn>
1155  <iref item="Cache Directives" primary="true" subitem="private" />
1156  <iref item="private" primary="true" subitem="Cache Directive" />
1157  <list>
1158    <t>The private response directive indicates that the response message is intended for
1159      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1160      cache &MAY; store the response.</t>
1161    <t>If the private response directive specifies one or more field-names, this
1162      requirement is limited to the field-values associated with the listed response
1163      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1164      cache, whereas the remainder of the response message &MAY; be.</t>
1165    <t>
1166      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1167      be stored, and cannot ensure the privacy of the message content.
1168      Also, private response directives with field-names are often handled by
1169      implementations as if an unqualified private directive was received; i.e.,
1170      the special handling for the qualified form is not widely implemented.</t>
1171  </list>
1172</t>
1173<t>
1174  <x:dfn>no-cache</x:dfn>
1175  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1176  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1177  <list>
1178    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1179      satisfy a subsequent request without successful validation on the origin server.
1180      This allows an origin server to prevent caching even by caches that have been
1181      configured to return stale responses.</t>
1182    <t>If the no-cache response directive specifies one or more field-names, this
1183      requirement is limited to the field-values associated with the listed response
1184      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1185      to a subsequent request without successful validation on the origin server. This
1186      allows an origin server to prevent the re-use of certain header fields in a
1187      response, while still allowing caching of the rest of the response.</t>
1188    <t>
1189      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1190      Also, no-cache response directives with field-names are often handled by
1191      implementations as if an unqualified no-cache directive was received; i.e.,
1192      the special handling for the qualified form is not widely implemented.
1193    </t>
1194  </list>
1195</t>
1196<t>
1197  <x:dfn>no-store</x:dfn>
1198  <iref item="Cache Directives" primary="true" subitem="no-store" />
1199  <iref item="no-store" primary="true" subitem="Cache Directive" />
1200  <list>
1201    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1202      part of either the immediate request or response. This directive applies to both
1203      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1204      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1205      and &MUST; make a best-effort attempt to remove the information from volatile
1206      storage as promptly as possible after forwarding it.</t>
1207    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1208      particular, malicious or compromised caches might not recognize or obey this
1209      directive, and communications networks may be vulnerable to eavesdropping.</t>
1210  </list>
1211</t>
1212<t>
1213  <x:dfn>must-revalidate</x:dfn>
1214  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1215  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1216  <list>
1217    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1218     used to satisfy subsequent requests without successful validation on the origin server.</t>
1219    <t>The must-revalidate directive is necessary to support reliable operation for
1220      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1221      the must-revalidate directive; in particular, if the cache cannot reach the origin
1222      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1223    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1224      validate a request on the entity could result in incorrect operation, such as a
1225      silently unexecuted financial transaction.</t>
1226  </list>
1227</t>
1228<t>
1229  <x:dfn>proxy-revalidate</x:dfn>
1230  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1231  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1232  <list>
1233    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1234      response directive, except that it does not apply to non-shared caches.</t>
1235  </list>
1236</t>
1237<t>
1238  <x:dfn>max-age</x:dfn>
1239  <iref item="Cache Directives" primary="true" subitem="max-age" />
1240  <iref item="max-age" primary="true" subitem="Cache Directive" />
1241  <list>
1242    <t>The max-age response directive indicates that response is to be considered stale
1243      after its age is greater than the specified number of seconds.</t>
1244  </list>
1245</t>
1246<t>
1247  <x:dfn>s-maxage</x:dfn>
1248  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1249  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1250  <list>
1251    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1252      specified by this directive overrides the maximum age specified by either the
1253      max-age directive or the Expires header. The s-maxage directive also implies the
1254      semantics of the proxy-revalidate response directive.</t>
1255  </list>
1256</t>
1257<t>
1258  <x:dfn>no-transform</x:dfn>
1259  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1260  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1261  <list>
1262    <t>The no-transform response directive indicates that an intermediate cache or proxy
1263      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1264      headers, nor the response entity-body.</t>
1265  </list>
1266</t>
1267
1268</section>
1269
1270<section anchor="cache.control.extensions" title="Cache Control Extensions">
1271<t>
1272  The Cache-Control header field can be extended through the use of one or more
1273  cache-extension tokens, each with an optional value. Informational extensions (those
1274  that do not require a change in cache behavior) can be added without changing the
1275  semantics of other directives. Behavioral extensions are designed to work by acting as
1276  modifiers to the existing base of cache directives. Both the new directive and the
1277  standard directive are supplied, such that applications that do not understand the new
1278  directive will default to the behavior specified by the standard directive, and those
1279  that understand the new directive will recognize it as modifying the requirements
1280  associated with the standard directive. In this way, extensions to the cache-control
1281  directives can be made without requiring changes to the base protocol.
1282</t>
1283<t>
1284  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1285  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1286  all directives that it does not understand.
1287</t>
1288<t>
1289  For example, consider a hypothetical new response directive called "community" that
1290  acts as a modifier to the private directive. We define this new directive to mean that,
1291  in addition to any non-shared cache, any cache that is shared only by members of the
1292  community named within its value may cache the response. An origin server wishing to
1293  allow the UCI community to use an otherwise private response in their shared cache(s)
1294  could do so by including
1295</t>
1296<figure><artwork type="example">
1297  Cache-Control: private, community="UCI"
1298</artwork></figure>
1299<t>
1300  A cache seeing this header field will act correctly even if the cache does not
1301  understand the community cache-extension, since it will also see and understand the
1302  private directive and thus default to the safe behavior.
1303</t>
1304<t>
1305  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1306  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1307  directives (or the response's default cacheability) such that the cache behavior will
1308  remain minimally correct even if the cache does not understand the extension(s).
1309</t>
1310<t>
1311  The HTTP Cache Directive Registry defines the name space for the cache
1312  directives.
1313</t>
1314<t>
1315  Registrations &MUST; include the following fields:
1316  <list style="symbols">
1317    <t>Cache Directive Name</t>
1318    <t>Pointer to specification text</t>
1319  </list>
1320</t>
1321<t>
1322  Values to be added to this name space are subject to IETF review
1323  (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
1324</t>
1325<t>
1326  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1327</t>
1328</section>
1329
1330</section>
1331
1332<section anchor="header.expires" title="Expires">
1333  <iref item="Expires header" primary="true" x:for-anchor="" />
1334  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1335  <x:anchor-alias value="Expires"/>
1336  <x:anchor-alias value="Expires-v"/>
1337<t>
1338  The "Expires" entity-header field gives the date/time after which the response is
1339  considered stale. See <xref target="expiration.model" /> for further discussion of the
1340  freshness model.
1341</t>
1342<t>
1343  The presence of an Expires field does not imply that the original resource will change or
1344  cease to exist at, before, or after that time.
1345</t>
1346<t>
1347  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1348  it &MUST; be sent in rfc1123-date format.
1349</t>
1350<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1351  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1352  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1353</artwork></figure>
1354<figure>
1355  <preamble>For example</preamble>
1356<artwork type="example">
1357  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1358</artwork></figure>
1359<x:note>
1360  <t>
1361    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1362    directive (see <xref target="cache-response-directive" />), that directive overrides
1363    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1364  </t>
1365</x:note>
1366<t>
1367  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1368</t>
1369<t>
1370  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1371  including the value "0", as in the past (i.e., "already expired").
1372</t>
1373</section>
1374
1375<section anchor="header.pragma" title="Pragma">
1376  <iref item="Pragma header" primary="true" x:for-anchor="" />
1377  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1378  <x:anchor-alias value="extension-pragma"/>
1379  <x:anchor-alias value="Pragma"/>
1380  <x:anchor-alias value="Pragma-v"/>
1381  <x:anchor-alias value="pragma-directive"/>
1382<t>
1383  The "Pragma" general-header field is used to include implementation-specific directives
1384  that might apply to any recipient along the request/response chain. All pragma directives
1385  specify optional behavior from the viewpoint of the protocol; however, some systems
1386  &MAY; require that behavior be consistent with the directives.
1387</t>
1388<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1389  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1390  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1391  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1392  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1393</artwork></figure>
1394<t>
1395  When the no-cache directive is present in a request message, an application &SHOULD;
1396  forward the request toward the origin server even if it has a cached copy of what is being
1397  requested. This pragma directive has the same semantics as the no-cache response directive
1398  (see <xref target="cache-response-directive" />) and is defined here for backward
1399  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1400  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1401  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1402</t>
1403<x:note>
1404  <t>
1405    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1406    is not actually specified, it does not provide a reliable replacement for
1407    "Cache-Control: no-cache" in a response.
1408  </t>
1409</x:note>
1410<t>
1411  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1412</t>
1413</section>
1414
1415<section anchor="header.vary" title="Vary">
1416  <iref item="Vary header" primary="true" x:for-anchor="" />
1417  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1418  <x:anchor-alias value="Vary"/>
1419  <x:anchor-alias value="Vary-v"/>
1420<t>
1421  The "Vary" response-header field conveys the set of request-header fields
1422  that were used to select the representation.
1423</t>
1424<t>
1425  Caches use this information, in part, to determine whether a stored response
1426  can be used to satisfy a given request; see
1427  <xref target="caching.negotiated.responses" />.
1428  determines, while the response is fresh, whether a cache is permitted to use the
1429  response to reply to a subsequent request without validation; see <xref 
1430  target="caching.negotiated.responses" />.
1431</t>
1432<t>
1433  In uncacheable or stale responses, the Vary field value advises the user agent about
1434  the criteria that were used to select the representation.
1435</t>
1436<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1437  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1438  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1439</artwork></figure>
1440<t>
1441  The set of header fields named by the Vary field value is known as the selecting
1442  request-headers.
1443</t>
1444<t>
1445  Servers &SHOULD; include a Vary header field with any cacheable response that is
1446  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1447  requests on that resource and informs the user agent about the presence of negotiation on
1448  that resource. A server &MAY; include a Vary header field with a non-cacheable
1449  response that is subject to server-driven negotiation, since this might provide the user
1450  agent with useful information about the dimensions over which the response varies at the
1451  time of the response.
1452</t>
1453<t>
1454  A Vary field value of "*" signals that unspecified parameters not limited to the
1455  request-headers (e.g., the network address of the client), play a role in the selection of
1456  the response representation; therefore, a cache cannot determine whether this response is
1457  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1458  it may only be generated by an origin server.
1459</t>
1460<t>
1461  The field-names given are not limited to the set of standard request-header fields
1462  defined by this specification. Field names are case-insensitive.
1463</t>
1464</section>
1465
1466<section anchor="header.warning" title="Warning">
1467  <iref item="Warning header" primary="true" x:for-anchor="" />
1468  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1469  <x:anchor-alias value="Warning"/>
1470  <x:anchor-alias value="Warning-v"/>
1471  <x:anchor-alias value="warning-value"/>
1472  <x:anchor-alias value="warn-agent"/>
1473  <x:anchor-alias value="warn-code"/>
1474  <x:anchor-alias value="warn-date"/>
1475  <x:anchor-alias value="warn-text"/>
1476<t>
1477  The "Warning" general-header field is used to carry additional information about the status
1478  or transformation of a message that might not be reflected in the message. This
1479  information is typically used to warn about possible incorrectness introduced by caching
1480  operations or transformations applied to the entity body of the message.
1481</t>
1482<t>
1483  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1484  warning, rather than an error status code, distinguishes these responses from true failures.
1485</t>
1486<t>
1487  Warning headers can in general be applied to any message, however some warn-codes are
1488  specific to caches and can only be applied to response messages.
1489</t>
1490<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1491  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1492  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1493 
1494  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
1495                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1496 
1497  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1498  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1499                  ; the name or pseudonym of the server adding
1500                  ; the Warning header, for use in debugging
1501  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1502  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1503</artwork></figure>
1504<t>
1505  Multiple warnings can be attached to a response (either by the origin server or by
1506  a cache), including multiple warnings with the same code number, only differing
1507  in warn-text.
1508</t>
1509<t>
1510  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1511  possible, in the order that they appear in the response.
1512</t>
1513<t>
1514  Systems that generate multiple Warning headers &SHOULD; order them with this user
1515  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1516  Warning headers.
1517</t>
1518<t>
1519  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1520  Warning is required to be deleted from a stored response after validation:
1521  <list style="symbols">
1522    <t>1xx Warnings describe the freshness or validation status of the response, and so
1523      &MUST; be deleted by caches after validation. They can only be generated by a cache
1524      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1525    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1526      not rectified by a validation (for example, a lossy compression of the entity bodies)
1527      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1528      returned, in which case they &MUST; be.</t>
1529  </list>
1530</t>
1531<t>
1532  If an implementation sends a message with one or more Warning headers to a receiver whose
1533  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1534  warn-date that matches the Date header in the message.
1535</t>
1536<t>
1537  If an implementation receives a message with a warning-value that includes a warn-date,
1538  and that warn-date is different from the Date value in the response, then that
1539  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1540  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1541  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1542  well.
1543</t>
1544<t>
1545  The following warn-codes are defined by this specification, each with a recommended
1546  warn-text in English, and a description of its meaning.
1547</t>
1548<t><?rfc needLines="4"?>
1549  110 Response is stale
1550  <list>
1551    <t>&SHOULD; be included whenever the returned response is stale.</t>
1552  </list>
1553</t>
1554<t><?rfc needLines="4"?>
1555  111 Revalidation failed
1556  <list>
1557    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1558      validate the response failed, due to an inability to reach the server.</t>
1559  </list>
1560</t>
1561<t><?rfc needLines="4"?>
1562  112 Disconnected operation
1563  <list>
1564    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1565      the network for a period of time.</t>
1566  </list>
1567</t>
1568<t><?rfc needLines="4"?>
1569  113 Heuristic expiration
1570  <list>
1571    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1572      greater than 24 hours and the response's age is greater than 24 hours.</t>
1573  </list>
1574</t>
1575<t><?rfc needLines="4"?>
1576  199 Miscellaneous warning
1577  <list>
1578    <t>The warning text can include arbitrary information to be presented to a human
1579      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1580      action, besides presenting the warning to the user.</t>
1581  </list>
1582</t>
1583<t><?rfc needLines="4"?>
1584  214 Transformation applied
1585  <list>
1586    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1587      transformation changing the content-coding (as specified in the Content-Encoding
1588      header) or media-type (as specified in the Content-Type header) of the response, or
1589      the entity-body of the response, unless this Warning code already appears in the
1590      response.</t>
1591  </list>
1592</t>
1593<t><?rfc needLines="4"?>
1594  299 Miscellaneous persistent warning
1595  <list>
1596    <t>The warning text can include arbitrary information to be presented to a human
1597      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1598      action.</t>
1599  </list>
1600</t>
1601</section>
1602
1603</section>
1604
1605<section anchor="history.lists" title="History Lists">
1606<t>
1607  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1608  can be used to redisplay an entity retrieved earlier in a session.
1609</t>
1610<t>
1611  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1612  a history mechanism can display a previous representation even if it has expired.
1613</t>
1614  <t>
1615  This does not prohibit the history mechanism from telling the user that a
1616  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1617  </t>
1618</section>
1619
1620
1621<section anchor="IANA.considerations" title="IANA Considerations">
1622
1623<section title="Cache Directive Registry" anchor="cache.directive.registration">
1624<t>
1625  The registration procedure for HTTP Cache Directives is defined by
1626  <xref target="cache.control.extensions"/> of this document.
1627</t>
1628<t>
1629   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
1630   and be populated with the registrations below:
1631</t>
1632<?BEGININC p6-cache.cache-directives ?>
1633<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1634<texttable xmlns:my="#my" align="left" suppress-title="true"
1635           anchor="iana.cache.directive.registration.table">
1636   <ttcol>Cache Directive</ttcol>
1637   <ttcol>Reference</ttcol>
1638
1639   <c>max-age</c>
1640   <c>
1641      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1642   </c>
1643   <c>max-stale</c>
1644   <c>
1645      <xref target="cache-request-directive"/>
1646   </c>
1647   <c>min-fresh</c>
1648   <c>
1649      <xref target="cache-request-directive"/>
1650   </c>
1651   <c>must-revalidate</c>
1652   <c>
1653      <xref target="cache-response-directive"/>
1654   </c>
1655   <c>no-cache</c>
1656   <c>
1657      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1658   </c>
1659   <c>no-store</c>
1660   <c>
1661      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1662   </c>
1663   <c>no-transform</c>
1664   <c>
1665      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1666   </c>
1667   <c>only-if-cached</c>
1668   <c>
1669      <xref target="cache-request-directive"/>
1670   </c>
1671   <c>private</c>
1672   <c>
1673      <xref target="cache-response-directive"/>
1674   </c>
1675   <c>proxy-revalidate</c>
1676   <c>
1677      <xref target="cache-response-directive"/>
1678   </c>
1679   <c>public</c>
1680   <c>
1681      <xref target="cache-response-directive"/>
1682   </c>
1683   <c>s-maxage</c>
1684   <c>
1685      <xref target="cache-response-directive"/>
1686   </c>
1687   <c>stale-if-error</c>
1688   <c>
1689      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1690   </c>
1691   <c>stale-while-revalidate</c>
1692   <c>
1693      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1694   </c>
1695</texttable>
1696<!--(END)-->
1697<?ENDINC p6-cache.cache-directives ?>
1698</section>
1699
1700<section anchor="message.header.registration" title="Message Header Registration">
1701<t>
1702  The Message Header Registry located at <eref
1703  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1704  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1705</t>
1706<?BEGININC p6-cache.iana-headers ?>
1707<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1708<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1709   <ttcol>Header Field Name</ttcol>
1710   <ttcol>Protocol</ttcol>
1711   <ttcol>Status</ttcol>
1712   <ttcol>Reference</ttcol>
1713
1714   <c>Age</c>
1715   <c>http</c>
1716   <c>standard</c>
1717   <c>
1718      <xref target="header.age"/>
1719   </c>
1720   <c>Cache-Control</c>
1721   <c>http</c>
1722   <c>standard</c>
1723   <c>
1724      <xref target="header.cache-control"/>
1725   </c>
1726   <c>Expires</c>
1727   <c>http</c>
1728   <c>standard</c>
1729   <c>
1730      <xref target="header.expires"/>
1731   </c>
1732   <c>Pragma</c>
1733   <c>http</c>
1734   <c>standard</c>
1735   <c>
1736      <xref target="header.pragma"/>
1737   </c>
1738   <c>Vary</c>
1739   <c>http</c>
1740   <c>standard</c>
1741   <c>
1742      <xref target="header.vary"/>
1743   </c>
1744   <c>Warning</c>
1745   <c>http</c>
1746   <c>standard</c>
1747   <c>
1748      <xref target="header.warning"/>
1749   </c>
1750</texttable>
1751<!--(END)-->
1752<?ENDINC p6-cache.iana-headers ?>
1753<t>
1754  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1755</t>
1756</section>
1757
1758</section>
1759
1760<section anchor="security.considerations" title="Security Considerations">
1761<t>
1762  Caches expose additional potential vulnerabilities, since the contents of the cache
1763  represent an attractive target for malicious exploitation. Because cache contents persist
1764  after an HTTP request is complete, an attack on the cache can reveal information long after
1765  a user believes that the information has been removed from the network. Therefore, cache
1766  contents should be protected as sensitive information.
1767</t>
1768</section>
1769
1770<section anchor="ack" title="Acknowledgments">
1771<t>
1772  Much of the content and presentation of the caching design is due to suggestions and
1773  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1774  and Larry Masinter.
1775</t>
1776</section>
1777
1778</middle>
1779
1780<back>
1781<references title="Normative References">
1782
1783  <reference anchor="Part1">
1784    <front>
1785      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1786      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1787        <organization abbrev="Day Software">Day Software</organization>
1788        <address><email>fielding@gbiv.com</email></address>
1789      </author>
1790      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1791        <organization>One Laptop per Child</organization>
1792        <address><email>jg@laptop.org</email></address>
1793      </author>
1794      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1795        <organization abbrev="HP">Hewlett-Packard Company</organization>
1796        <address><email>JeffMogul@acm.org</email></address>
1797      </author>
1798      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1799        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1800        <address><email>henrikn@microsoft.com</email></address>
1801      </author>
1802      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1803        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1804        <address><email>LMM@acm.org</email></address>
1805      </author>
1806      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1807        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1808        <address><email>paulle@microsoft.com</email></address>
1809      </author>
1810      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1811        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1812        <address><email>timbl@w3.org</email></address>
1813      </author>
1814      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1815        <organization abbrev="W3C">World Wide Web Consortium</organization>
1816        <address><email>ylafon@w3.org</email></address>
1817      </author>
1818      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1819        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1820        <address><email>julian.reschke@greenbytes.de</email></address>
1821      </author>
1822      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1823    </front>
1824    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1825    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1826  </reference>
1827
1828  <reference anchor="Part2">
1829    <front>
1830      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1831      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1832        <organization abbrev="Day Software">Day Software</organization>
1833        <address><email>fielding@gbiv.com</email></address>
1834      </author>
1835      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1836        <organization>One Laptop per Child</organization>
1837        <address><email>jg@laptop.org</email></address>
1838      </author>
1839      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1840        <organization abbrev="HP">Hewlett-Packard Company</organization>
1841        <address><email>JeffMogul@acm.org</email></address>
1842      </author>
1843      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1844        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1845        <address><email>henrikn@microsoft.com</email></address>
1846      </author>
1847      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1848        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1849        <address><email>LMM@acm.org</email></address>
1850      </author>
1851      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1852        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1853        <address><email>paulle@microsoft.com</email></address>
1854      </author>
1855      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1856        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1857        <address><email>timbl@w3.org</email></address>
1858      </author>
1859      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1860        <organization abbrev="W3C">World Wide Web Consortium</organization>
1861        <address><email>ylafon@w3.org</email></address>
1862      </author>
1863      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1864        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1865        <address><email>julian.reschke@greenbytes.de</email></address>
1866      </author>
1867      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1868    </front>
1869    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1870    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1871  </reference>
1872
1873  <reference anchor="Part4">
1874    <front>
1875      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1876      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1877        <organization abbrev="Day Software">Day Software</organization>
1878        <address><email>fielding@gbiv.com</email></address>
1879      </author>
1880      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1881        <organization>One Laptop per Child</organization>
1882        <address><email>jg@laptop.org</email></address>
1883      </author>
1884      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1885        <organization abbrev="HP">Hewlett-Packard Company</organization>
1886        <address><email>JeffMogul@acm.org</email></address>
1887      </author>
1888      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1889        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1890        <address><email>henrikn@microsoft.com</email></address>
1891      </author>
1892      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1893        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1894        <address><email>LMM@acm.org</email></address>
1895      </author>
1896      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1897        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1898        <address><email>paulle@microsoft.com</email></address>
1899      </author>
1900      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1901        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1902        <address><email>timbl@w3.org</email></address>
1903      </author>
1904      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1905        <organization abbrev="W3C">World Wide Web Consortium</organization>
1906        <address><email>ylafon@w3.org</email></address>
1907      </author>
1908      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1909        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1910        <address><email>julian.reschke@greenbytes.de</email></address>
1911      </author>
1912      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1913    </front>
1914    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1915    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1916  </reference>
1917
1918  <reference anchor="Part5">
1919    <front>
1920      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1921      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1922        <organization abbrev="Day Software">Day Software</organization>
1923        <address><email>fielding@gbiv.com</email></address>
1924      </author>
1925      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1926        <organization>One Laptop per Child</organization>
1927        <address><email>jg@laptop.org</email></address>
1928      </author>
1929      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1930        <organization abbrev="HP">Hewlett-Packard Company</organization>
1931        <address><email>JeffMogul@acm.org</email></address>
1932      </author>
1933      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1934        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1935        <address><email>henrikn@microsoft.com</email></address>
1936      </author>
1937      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1938        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1939        <address><email>LMM@acm.org</email></address>
1940      </author>
1941      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1942        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1943        <address><email>paulle@microsoft.com</email></address>
1944      </author>
1945      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1946        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1947        <address><email>timbl@w3.org</email></address>
1948      </author>
1949      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1950        <organization abbrev="W3C">World Wide Web Consortium</organization>
1951        <address><email>ylafon@w3.org</email></address>
1952      </author>
1953      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1954        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1955        <address><email>julian.reschke@greenbytes.de</email></address>
1956      </author>
1957      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1958    </front>
1959    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1960    <x:source basename="p5-range" href="p5-range.xml" />
1961  </reference>
1962
1963  <reference anchor="Part7">
1964    <front>
1965      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1966      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1967        <organization abbrev="Day Software">Day Software</organization>
1968        <address><email>fielding@gbiv.com</email></address>
1969      </author>
1970      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1971        <organization>One Laptop per Child</organization>
1972        <address><email>jg@laptop.org</email></address>
1973      </author>
1974      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1975        <organization abbrev="HP">Hewlett-Packard Company</organization>
1976        <address><email>JeffMogul@acm.org</email></address>
1977      </author>
1978      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1979        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1980        <address><email>henrikn@microsoft.com</email></address>
1981      </author>
1982      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1983        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1984        <address><email>LMM@acm.org</email></address>
1985      </author>
1986      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1987        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1988        <address><email>paulle@microsoft.com</email></address>
1989      </author>
1990      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1991        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1992        <address><email>timbl@w3.org</email></address>
1993      </author>
1994      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1995        <organization abbrev="W3C">World Wide Web Consortium</organization>
1996        <address><email>ylafon@w3.org</email></address>
1997      </author>
1998      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1999        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2000        <address><email>julian.reschke@greenbytes.de</email></address>
2001      </author>
2002      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2003    </front>
2004    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2005    <x:source basename="p7-auth" href="p7-auth.xml" />
2006  </reference>
2007
2008  <reference anchor="RFC2119">
2009    <front>
2010      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2011      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2012        <organization>Harvard University</organization>
2013        <address><email>sob@harvard.edu</email></address>
2014      </author>
2015      <date month="March" year="1997" />
2016    </front>
2017    <seriesInfo name="BCP" value="14" />
2018    <seriesInfo name="RFC" value="2119" />
2019  </reference>
2020
2021  <reference anchor="RFC5234">
2022    <front>
2023      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2024      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2025        <organization>Brandenburg InternetWorking</organization>
2026        <address>
2027          <email>dcrocker@bbiw.net</email>
2028        </address> 
2029      </author>
2030      <author initials="P." surname="Overell" fullname="Paul Overell">
2031        <organization>THUS plc.</organization>
2032        <address>
2033          <email>paul.overell@thus.net</email>
2034        </address>
2035      </author>
2036      <date month="January" year="2008"/>
2037    </front>
2038    <seriesInfo name="STD" value="68"/>
2039    <seriesInfo name="RFC" value="5234"/>
2040  </reference>
2041 
2042</references>
2043
2044<references title="Informative References">
2045
2046  <reference anchor="RFC1305">
2047    <front>
2048      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2049      <author fullname="David L. Mills" initials="D." surname="Mills">
2050        <organization>University of Delaware, Electrical Engineering Department</organization>
2051        <address><email>mills@udel.edu</email></address>
2052      </author>
2053      <date month="March" year="1992" />
2054    </front>
2055    <seriesInfo name="RFC" value="1305" />
2056  </reference>
2057
2058  <reference anchor="RFC2616">
2059    <front>
2060      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2061      <author fullname="R. Fielding" initials="R." surname="Fielding">
2062        <organization>University of California, Irvine</organization>
2063        <address><email>fielding@ics.uci.edu</email></address>
2064      </author>
2065      <author fullname="J. Gettys" initials="J." surname="Gettys">
2066        <organization>W3C</organization>
2067        <address><email>jg@w3.org</email></address>
2068      </author>
2069      <author fullname="J. Mogul" initials="J." surname="Mogul">
2070        <organization>Compaq Computer Corporation</organization>
2071        <address><email>mogul@wrl.dec.com</email></address>
2072      </author>
2073      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2074        <organization>MIT Laboratory for Computer Science</organization>
2075        <address><email>frystyk@w3.org</email></address>
2076      </author>
2077      <author fullname="L. Masinter" initials="L." surname="Masinter">
2078        <organization>Xerox Corporation</organization>
2079        <address><email>masinter@parc.xerox.com</email></address>
2080      </author>
2081      <author fullname="P. Leach" initials="P." surname="Leach">
2082        <organization>Microsoft Corporation</organization>
2083        <address><email>paulle@microsoft.com</email></address>
2084      </author>
2085      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2086        <organization>W3C</organization>
2087        <address><email>timbl@w3.org</email></address>
2088      </author>
2089      <date month="June" year="1999" />
2090    </front>
2091    <seriesInfo name="RFC" value="2616" />
2092  </reference>
2093
2094  <reference anchor="RFC3864">
2095    <front>
2096      <title>Registration Procedures for Message Header Fields</title>
2097      <author fullname="G. Klyne" initials="G." surname="Klyne">
2098        <organization>Nine by Nine</organization>
2099        <address><email>GK-IETF@ninebynine.org</email></address>
2100      </author>
2101      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2102        <organization>BEA Systems</organization>
2103        <address><email>mnot@pobox.com</email></address>
2104      </author>
2105      <author fullname="J. Mogul" initials="J." surname="Mogul">
2106        <organization>HP Labs</organization>
2107        <address><email>JeffMogul@acm.org</email></address>
2108      </author>
2109      <date month="September" year="2004" />
2110    </front>
2111    <seriesInfo name="BCP" value="90" />
2112    <seriesInfo name="RFC" value="3864" />
2113  </reference>
2114
2115  <reference anchor='RFC5226'>
2116    <front>
2117      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2118      <author initials='T.' surname='Narten' fullname='T. Narten'>
2119        <organization>IBM</organization>
2120        <address><email>narten@us.ibm.com</email></address>
2121      </author>
2122      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2123        <organization>Google</organization>
2124        <address><email>Harald@Alvestrand.no</email></address>
2125      </author>
2126      <date year='2008' month='May' />
2127    </front>
2128    <seriesInfo name='BCP' value='26' />
2129    <seriesInfo name='RFC' value='5226' />
2130  </reference>
2131
2132  <reference anchor='RFC5861'>
2133    <front>
2134      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2135      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2136        <organization>Yahoo! Inc.</organization>
2137        <address><email>mnot@yahoo-inc.com</email></address>
2138      </author>
2139      <date month="April" year="2010"/>
2140    </front>
2141    <seriesInfo name='RFC' value='5861' />
2142  </reference>
2143
2144</references>
2145
2146<section anchor="compatibility" title="Compatibility with Previous Versions">
2147
2148<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2149<t>
2150  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2151  this missing case.
2152  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2153</t>
2154<t>
2155  Range request responses would become very verbose if all meta-data were always returned;
2156  by allowing the server to only send needed headers in a 206 response, this problem can be
2157  avoided.
2158  (<xref target="combining.headers" />)
2159</t>
2160<t>
2161  The Cache-Control: max-age directive was not properly defined for responses.
2162  (<xref target="cache-response-directive"/>)
2163</t>
2164<t>
2165  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2166    format="counter" target="expiration.model" />, <xref format="counter"
2167    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2168  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2169  header, as PUT or other methods may have need for it in requests.
2170</t>
2171</section>
2172
2173<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2174<t>
2175  Make the specified age calculation algorithm less conservative.
2176  (<xref target="age.calculations"/>)
2177</t>
2178<t>
2179  Remove requirement to consider Content-Location in successful responses
2180  in order to determine the appropriate response to use.
2181  (<xref target="validation.model" />)
2182</t>
2183<t>
2184  Clarify denial of service attack avoidance requirement.
2185  (<xref target="invalidation.after.updates.or.deletions" />)
2186</t>
2187<t>
2188  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2189  anymore, as these aspects never were implemented.
2190  (<xref target="header.warning" />)
2191</t>
2192</section>
2193
2194    </section>
2195
2196<?BEGININC p6-cache.abnf-appendix ?>
2197<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2198<figure>
2199<artwork type="abnf" name="p6-cache.parsed-abnf">
2200<x:ref>Age</x:ref> = "Age:" OWS Age-v
2201<x:ref>Age-v</x:ref> = delta-seconds
2202
2203<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2204<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2205 cache-directive ] )
2206
2207<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2208<x:ref>Expires-v</x:ref> = HTTP-date
2209
2210<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2211
2212<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2213
2214<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2215<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2216 pragma-directive ] )
2217
2218<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2219<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2220 ] ) )
2221
2222<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2223<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2224 ] )
2225
2226<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2227<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2228<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2229 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2230 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2231 cache-extension
2232<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2233 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2234 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2235 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2236 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2237 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2238
2239<x:ref>delta-seconds</x:ref> = 1*DIGIT
2240
2241<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2242
2243<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2244
2245<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2246<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2247<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2248
2249<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2250
2251<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2252
2253<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2254
2255<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2256<x:ref>warn-code</x:ref> = 3DIGIT
2257<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2258<x:ref>warn-text</x:ref> = quoted-string
2259<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2260 ]
2261</artwork>
2262</figure>
2263<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2264; Age defined but not used
2265; Cache-Control defined but not used
2266; Expires defined but not used
2267; Pragma defined but not used
2268; Vary defined but not used
2269; Warning defined but not used
2270</artwork></figure></section>
2271<?ENDINC p6-cache.abnf-appendix ?>
2272
2273<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2274
2275<section title="Since RFC2616">
2276  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2277</section>
2278
2279<section title="Since draft-ietf-httpbis-p6-cache-00">
2280<t>
2281  Closed issues:
2282  <list style="symbols">
2283    <t>
2284      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2285    <t>
2286      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
2287    <t>
2288      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2289    <t>
2290      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2291    <t>
2292      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2293    <t>
2294      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2295    <t>
2296      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2297    <t>
2298      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2299    <t>
2300      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2301  </list>
2302</t>
2303<t>
2304  Other changes:
2305  <list style="symbols">
2306    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2307        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2308  </list>
2309</t>
2310</section>
2311
2312<section title="Since draft-ietf-httpbis-p6-cache-01">
2313<t>
2314  Closed issues:
2315  <list style="symbols">
2316    <t>
2317      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2318  </list>
2319</t>
2320<t>
2321  Other changes:
2322  <list style="symbols">
2323    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2324      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2325    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2326      specification.</t>
2327  </list>
2328</t>
2329</section>
2330
2331<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2332<t>
2333  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2334  <list style="symbols">
2335    <t>Reference RFC 3984, and update header registrations for headers defined in this
2336      document.</t>
2337  </list>
2338</t>
2339</section>
2340
2341<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2342<t>
2343  Closed issues:
2344  <list style="symbols">
2345    <t>
2346      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2347  </list>
2348</t>
2349</section>
2350
2351<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2352<t>
2353  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2354  <list style="symbols"> 
2355    <t>
2356      Use "/" instead of "|" for alternatives.
2357    </t>
2358    <t>
2359      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2360      whitespace ("OWS") and required whitespace ("RWS").
2361    </t>
2362    <t>
2363      Rewrite ABNFs to spell out whitespace rules, factor out
2364      header value format definitions.
2365    </t>
2366  </list>
2367</t>
2368</section>
2369
2370<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2371<t>
2372  This is a total rewrite of this part of the specification.
2373</t>
2374<t>
2375  Affected issues:
2376  <list style="symbols">
2377    <t>
2378      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2379    <t>
2380      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2381    <t>
2382      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2383    <t>
2384      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2385  </list>
2386</t>
2387<t>
2388  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2389  <list style="symbols"> 
2390    <t>
2391      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2392    </t>
2393  </list>
2394</t>
2395</section>
2396
2397<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2398<t>
2399  Closed issues:
2400  <list style="symbols"> 
2401    <t>
2402      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2403      "base for numeric protocol elements"
2404    </t>
2405  </list>
2406</t>
2407<t>
2408  Affected issues:
2409  <list style="symbols">
2410    <t>
2411      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2412      Vary and non-existant headers
2413    </t>
2414  </list>
2415</t>
2416</section>
2417
2418<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2419<t>
2420  Closed issues:
2421  <list style="symbols"> 
2422    <t>
2423      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2424      "Definition of 1xx Warn-Codes"
2425    </t>
2426    <t>
2427      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2428      "Content-Location on 304 responses"
2429    </t>
2430    <t>
2431      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2432      "private and no-cache CC directives with headers"
2433    </t>
2434    <t>
2435      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2436      "RFC2047 and warn-text"
2437    </t>
2438  </list>
2439</t>
2440</section>
2441
2442<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2443<t>
2444  Closed issues:
2445  <list style="symbols"> 
2446    <t>
2447      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2448      "serving negotiated responses from cache: header-specific canonicalization"
2449    </t>
2450    <t>
2451      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2452      "Effect of CC directives on history lists"
2453    </t>
2454  </list>
2455</t>
2456<t>
2457  Affected issues:
2458  <list style="symbols">
2459    <t>
2460      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2461      Status codes and caching
2462    </t>
2463  </list>
2464</t>
2465<t>
2466  Partly resolved issues:
2467  <list style="symbols"> 
2468    <t>
2469      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2470      "Placement of 13.5.1 and 13.5.2"
2471    </t>
2472  </list>
2473</t>
2474</section>
2475
2476<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2477<t>
2478  Closed issues:
2479  <list style="symbols"> 
2480    <t>
2481      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2482      "Age calculation"
2483    </t>
2484    <t>
2485      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2486      "Clarify differences between / requirements for request and response CC directives"
2487    </t>
2488        <t>
2489                <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2490                "Caching authenticated responses"
2491        </t>
2492    <t>
2493      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2494      "IANA registry for cache-control directives"
2495    </t>
2496    <t>
2497      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2498      "Heuristic caching of URLs with query components"
2499    </t>
2500  </list>
2501</t>
2502<t>
2503  Partly resolved issues:
2504  <list style="symbols"> 
2505    <t>
2506      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2507      "Term for the requested resource's URI"
2508    </t>
2509  </list>
2510</t>
2511</section>
2512
2513</section>
2514  </back>
2515</rfc>
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